Discussions on the world's shortage of drinking and irrigation water are commonplace. In some parts of the world whole cities are going to have to be abandoned because of prolonged drought.
The only inexhaustible supply of water is the sea but desalination of water in significant quantities to supply major population centers or large scale irrigation projects is costly. Many desalination plants operate on the basis of reverse osmosis. In this type of plant the water to be desalinated is forced through a semi-permeable membrane so that the dissolved solids are removed by the membrane. Other plants operate on the basis of evaporation.
A major problem with both the methods described is that the water obtained is, in the case of the evaporation method, pure distilled water, and in the reverse osmosis method is of the same degree of purity as distilled water. It has virtually all the minerals that were dissolved therein removed. Water without any calcium or magnesium in it is aggressive towards metal pipes and other metal objects with which it comes into contact. Hence these minerals must be added to the recovered water. Furthermore distilled water is tasteless and, being devoid of essential minerals, cannot be used for human consumption over a prolonged period. Hence, for drinking purposes, it is necessary to add a range of minerals to convert the water from "flat" distilled water to acceptable drinking water. In both methods described the essential minerals which were present in the sea water are in the brine which is a by-product of the process. A significant cost in producing water from either type of plant is thus the cost of the minerals which must be reintroduced into the water and the equipment needed for this purpose.
In an evaporation plant the power needed to evaporate the sea water is also significant when costs per megaliter of recovered water are calculated.
Reverse osmosis membranes are of composite construction and one extensively used form comprises two films of a complex polymeric resin which together define a salt passage. In the passage there is an element for inducing turbulence in the flow. The element is usually a welded mesh of plastics material filaments. A number of these membranes are wound in a complex manner onto a central tube. Water which passes through said films enters spaces between adjacent membranes and flows to the central tube. The tube has apertures in the wall thereof to permit the recovered water to enter the tube. The brine, that is, the residue of the sea water and the bulk of the dissolved solids flows out of the multitude of salt passages to waste or to a salt recovery plant.
It is accepted by those working in this art that on each side of each salt passage, and immediately adjacent each film, there is a concentration polarization layer. These layers, which are of multi-molecular thickness, contain a higher concentration of dissolved solids than the bulk flow in the part of the salt passage mid-way between the films. The turbulence inducing element is intended to reduce the thickness of the concentration polarization layer and hence enhance the ability of the membrane to allow water to permeate through it. Typically a state of the art reverse osmosis membrane will achieve a 99.3% dissolved solids rejection rate. The dissolved solids that pass through the membrane largely consist of common salt as its molecules are smaller than the molecules of most other minerals. A percentage of 0.7% represents 400-500 parts per million of dissolved solids in the recovered water, depending on the initial salinity of the sea water, and is below the threshold at which the dissolved solids impart taste to the water.
Fouling of reverse osmosis membranes is a major problem and measures which increase the cost of water production have to be taken to inhibit fouling and to remove it when it does occur. Fouling can result from mineral deposition in the membrane or from organic growth. By way of example, before the sea water reaches the membrane it is treated with an inhibitor such as sodium hexametaphosphate (known commonly as "shrimp"). This limits calcium and magnesium precipitation on the membrane in the form of calcium and magnesium carbonates but adds another factor to production costs.
Membrane manufacturers recommend a relatively low flux rate (rate of water flow through a membrane in litres per hour per square metre of membrane) to avoid rapid fouling. Back-washing of a membrane, that is, causing water to flow in the reverse direction through the salt passages, is a standard procedure for removing fouling. If a membrane is heavily fouled it must be removed from the recovery plant and subjected to a variety of treatments for the purpose of removing the fouling. In extreme cases the fouling cannot be removed and the membrane has to be discarded.
As a result of all these factors water produced from a reverse osmosis plant is more costly than water obtained by purifying water from a storage dam or river. Hence, despite the world's shortage of water, only a small percentage of the world's water is produced using reverse osmosis plants to desalinate sea water.